Trip systems are designed to respond to power faults detected in circuit breakers. Most simple trip systems employ an electromagnet to trip the circuit in response to short circuit or overload faults The electromagnet provides a magnetic field in response to the current flowing through the breaker. When the current level increases beyond a predetermined threshold, the magnetic field "trips" a mechanism which causes a set of circuit breaker contacts to release, thereby "breaking" the circuit path.
Many simple trip systems also employ a slower responding bi-metallic strip, which is useful for detecting a more subtle overload fault. This is because the extent of the strip's deflection represents an accurate thermal history of the circuit breaker and, therefore, even slight current overloads. Generally, the heat generated by the current overload will cause the bi-metallic strip to deflect into the tripping mechanism to break the circuit path.
The tripping systems discussed above are generally adequate for many simple circuit breaker applications, but there has been an increasing demand for a more intelligent and flexible tripping system. For example, many factories today include 3-phase power equipment which is often replaced or moved on a regular basis. Consequently, the circuit breaker tripping specifications, e.g., current thresholds, for that equipment must be adjusted Thus, processor-based tripping systems have been developed to provide user-programmable flexibility.
While adding flexibility, processor-based tripping systems have interrupted the current path in response to power faults using techniques that are inaccurate or unreliable under certain conditions. For example, processor based systems that are fault powered, i.e., powered from the current flowing through the circuit breaker, usually employ a solenoid to break the circuit breaker current path. Typically, it is only after a power fault is detected in the current path that the processor attempts to engage the solenoid. However, after a power fault, system power is sometimes insufficient to successfully engage the solenoid. Not only might the attempted engagement fail, it will further dissipate system power.
Prior art systems have avoided such reliability problems by including a separate power supply which is not susceptible to faults. Unfortunately, a separate power supply is not acceptable in many applications due to cost and maintenance problems.